1. Field of the Invention
The present invention relates to an optotype presenting apparatus which presents an optotype and/or a test chart for testing visual performance of an eye.
2. Description of Related Art
Conventionally, there is known an optotype presenting apparatus of a projection type such that an optotype and/or a test chart drawn on a chart disk is illuminated from behind so as to be enlarged and projected onto a screen at a distance for a test. In addition, there is known an optotype presenting apparatus of a display type such that an optotype and/or a test chart is shown on a display (see Japanese Patent Application Unexamined Publication No. 2006-42978).
In an optotype presenting apparatus of a display type, a technique for displaying a three-dimensional image can be used in order to present different optotypes to a right eye and a left eye of an examinee (see U.S. Pat. No. 5,638,082 corresponding to Japanese Patent Application Unexamined Publication No. Hei07-322304, and Japanese Patent Application Unexamined Publication No. 2002-311385). To be more specific, the different optotypes are presented to the right eye and the left eye by placing in front of the display a sheet-like polarization optical member which has two types of optical regions alternately arranged in a linear pattern or a grid pattern based on the size of pixels of the display and changes thereby light exiting from the display into light having polarizing axes perpendicular to each other when transmitting the light and by placing in front of the right eye and the left eye different polarization filters which have polarizing axes perpendicular to each other and coinciding with the polarizing axes of the light transmitted by the polarization optical member, allowing a binocular vision test to be performed.
Incidentally, as an example of a test chart used for the binocular vision test, a polarization red-green test chart is cited which is used for checking binocular balance and whether corrected refractive power is undercorrection or overcorrection (see Japanese Patent Application Unexamined Publication No. Hei05-253186).
FIGS. 7A to 7C are views showing one example of a polarization red-green test chart 100 which is used in the conventional optotype presenting apparatus of the projection type. In FIG. 7A, an optotype 101a at an upper portion and an optotype 101b at a left portion are green optotypes, and an optotype 101c at a lower portion and an optotype 101d at a right portion are red optotypes. Inside the optotypes 101a to 101d, black characters 102 are drawn. At the time of the test, different polarization filters having polarizing axes perpendicular to each other are placed in front of a right eye and a left eye of an examinee in order to enable a binocular balance test to be concurrently performed. In addition, in the conventional optotype presenting apparatus of the projection type, a polarization filter which has a polarizing axis coinciding with the polarizing axis of the polarization filter for the left eye is placed on a chart disk so that the optotypes 101b and 101d are visually perceived against a black background by the left eye as shown in FIG. 7B. Meanwhile, a polarization filter which has a polarizing axis coinciding with the polarizing axis of the polarization filter for the right eye is placed on the chart disk so that the optotypes 101a and 101c are visually perceived against a black background by the right eye as shown in FIG. 7C. When the examinee looks at the test chart 100 by both eyes, the optotypes 101a to 101d are visually perceived compositely against a black background as shown in FIG. 7A. Then, by checking which of the character 102 on the green optotypes or the character 102 on the red optotypes is more clearly visible, it is determined whether the corrected refractive power is overcorrection or undercorrection, and the binocular balance is concurrently checked.
Meanwhile, presentation of the polarization red-green test chart is performed by the conventional optotype presenting apparatus of the display type as follows.
By using a polarization element (vectograph film) having polarizing axes perpendicular to each other (see U.S. Pat. No. 5,638,082 corresponding to Japanese Patent Application Unexamined Publication No. Hei07-322304), or using a wave plate in addition to a polarization plate placed in front of a display (see Japanese Patent Application Unexamined Publication No. 2002-311385), the display of two types of the optical regions alternately arranged in the linear pattern based on the size of the pixels of the display is made so that some pixels of the display are displayed in black, the pixels corresponding to the optical regions not intended to be visually perceived by either of the eyes in a similar manner that the optotypes 101a to 101d are visually perceived against the black background as described above referring to FIGS. 7A to 7C. At this time, there is a gap between a light-emitting element of the display and the polarization optical member, so that a slight leak of light occurs while an attempt is carried out to present the different optotypes to the right eye and the left eye. To be more specific, because light through the optotypes 101a and 101c to be presented to the right eye leaks out, the optotypes 101a and 101c are visually perceived faintly in addition to the optotypes 101b and 101d to be presented to the left eye which are shown in FIG. 7B. Similarly, because light through the optotypes 101b and 101d to be presented to the left eye leaks out, the optotypes 101b and 101d are visually perceived faintly in addition to the optotypes 101a and 101c to be presented to the right eye which are shown in FIG. 7C. Because of this, a polarization red-green test with accuracy cannot be performed.